Typical examples of conventional practical methods for extrusion molding plastic sheets include an air knife method, a rubber roll touch method and a three-roll method.
However, if the three-roll method out of these methods is used, it is difficult to extrusion mold a plastic sheet into an extremely thin sheet having a thickness of not more than about 0.4 mm. As for the three methods, further description is omitted herein because these methods are only techniques relevant to the present invention and are not worthy of the prior art techniques from the viewpoint of patent.
In an apparatus wherein a plastic sheet extruded from a T-die is pressed by means of a pressure roll and a cooling roll facing each other to reduce the thickness of the plastic sheet, the pressure applied to the plastic sheet by the pressure roll must be increased to make the thickness of the resulting plastic sheet lower. However, the pressure roll and the cooling roll, each having a circular section, are brought into linear contact with the plastic sheet. Hence, if the pressure applied to the plastic sheet by the pressure roll is too high, various troubles such as stay of the molten resin on the upstream side take place. In the conventional apparatuses, therefore, the pressure applied to the plastic sheet was not able to be increased to such a degree to form an extremely thin plastic sheet. Further, the roll brought into linear contact with the plastic sheet in the softened or molten state cannot rapidly cool the plastic sheet sufficiently, and hence the resulting plastic sheet does not have desired characteristics and physical properties.
As apparatuses to solve these problems, those using a metallic belt schematically shown in FIG. 2A and FIG. 2B have been proposed. The apparatus shown in FIG. 2A includes a pressure roll 21A and a cooling roll 21B which face each other interposing therebetween a plastic sheet S extruded from a T-die 1, and further includes a corrosion-resistant (stainless) steel thin endless belt 31 wound around the pressure roll 21A, a support roll 22 and a press roll 23. The molten plastic sheet S extruded from the T-die 1 is brought into surface contact with the metallic belt 31 between the pressure roll 21A and the press roll 23 and with the (main) cooling roll 21B, and the sheet S is cooled.
The apparatus shown in FIG. 2B further includes a metallic endless belt 31B on the side of the main cooling roll 21B in addition to the metallic endless belt 31A provided on the side of the pressure roll 21A. That is, the apparatus of FIG. 2B includes a pressure roll 21A and a cooling roll 21B which face each other interposing therebetween a plastic sheet S extruded from a T-die 1, and further includes an endless belt 31 wound around the pressure roll 21A and a turning roll 22A, and an endless belt 31B wound around the cooling roll 21B and a turning roll 22B.
However, the apparatus shown in FIG. 2A necessarily becomes large-sized, and the apparatus shown in 2B becomes much more large-sized as compared with the apparatus of FIG. 2A. That is, in the apparatus of FIG. 2A, the metallic endless belt 31 is made to have an extremely small thickness (0.8 to 1.2 mm), but this belt cannot be bent in a large curvature because of stiffness of the metal used and the structure thereof, so that the diameters of the two rolls 21A and 22 must be made large. As a result, it becomes necessary to use large-diameter rolls 21A and 22 (and 21B according to circumstances) each having a diameter of not less than about 800 mm.
The apparatus of FIG. 2B has two lines of the metallic endless belts as described above, and therefore the size of this apparatus becomes much larger as compared with the apparatus having a single line of the metallic endless belt.
The endless belt used in such apparatuses is formed by joining both ends of a metallic belt, and the joint should be made so as to have no difference in level. Moreover, the joint should withstand flexures of very many times. For producing such a belt, extremely high technique is required, and therefore the production cost becomes high. Incidentally, most of the metallic endless belts favorably used for the molding apparatuses at present are imports.
Accordingly, the apparatuses using the metallic endless belts are associated with such problems that the large-sized apparatuses lower the degree of freedom in the choice of installation place, the installation cost is increased, and use of the metallic belt increases the production cost.